双面抛光机原理及结构(英文版).doc

The Development of the Precise Double Sided Polishing MachineThis project is supported by the Zhejiang science foundation (M503049), Zhejiang technical foundation (2004C21007), Zhejiang Education foundation (20030157), Ningbo technical foundation (2003A41031)Keywords: double sided polishing, photoelectron material, ultra-smooth surfaceAbstract. The functional materials such as sapphire, silicon wafer etc. are processed efficiently with ultra-smooth surface for the demand of the rapid development of the IC industry. The precise double sided polishing process is one of the main methods of getting the ultra-smooth surface for these materials. This paper introduces the structure and characteristic of the precise double sided polishing machine with a precise pressure control system equipped a new type electro-pneumatic digital servo valve. The works, such as machine design, development of the control system and optimization of process parameters etc. are carried out to meet the requirement of the precise double sided polishing machining process. This equipment has the characteristic of high machining precision and precise control capability, so it can be applied to the ultra-precise machining of the ultra-thin sapphire, silicon wafers etc.Introduction The ultra-smooth and flat surface on both sides of silicon and sapphire wafers can be got by double sided polishing process simultaneously and invariably, due to the workpiece runs in a complex motion in the carrier driven by the top and bottom polishing pads over the workpiece surface. This complex motion is intended to produce a homogenous polishing texture on the workpiece surface.Therefore, the polishing machine should has some characteristics , such as 1) powerful driving system and stronger structure, due to the big friction force between the workpiece and the pads; 2) smooth running and steady loading process to protect the ultra-thin wafers to be broken; 3) heat proof ability to avoid the thermal distortion of the machine body.A precise double sided polishing machine was developed based on the study about the machine design, loading and speeding control system with the software and hardware design, also some primary polishing parameters were discussed in this paper.Double sided polishing machineThe structure of double sided polishing machine. Fig.1 shows the double sided polishing machine developed. The double sided polishing machine is composed of some primary parts including driving system, pressure control system and electrical control system. The structure of the pressure control system is a gate type, the top and bottom plates, carries between those plates driven by a precise cylinder to achieve stable pressure control. And the driving system including powerful servo motors, belt and gear transmission, can provide more powerful and stable driving force compared with the ordinary lapping process. Fig.2 shows the structure of double sided polishing machine. Top plate, bottom plate, internal and external gear ring are driven by four motors respectively. Carriers which hold the workpieces (8) are driven by internal and external gear ring. Cylinder (6), which is on the top of the polishing machine, is used to make the top plate rise and drop under the control of computer in order to place the workpieces conveniently, and press workpiece during the polishing process. The top and bottom plates rotate at opposite direction with the same speed. The workpieces can move in the carriers due to their contact with the polishing pads. The complex motion of the wafers in the carriers is intended to produce a homogeneous polishing texture on the wafer surfaces 12. 1-botom polishing plate 2- ring gear 3-top polishing plate 4-polishing pad 5- center gear 6-air cylinder 7- pressure sensor 8-workpiece 9-carriers 10-belt 11-speed reducer 12- motor Fig.1 The double sided polishing machine Fig.2 Schematic of the double sided Polishing machineThe control of double sided polishing machine. Fig.3 shows the Schematic of electrical control system. Cylinder and servo motors are controlled by a computer control system. A pneumatic control system is used to control the polishing pressure during the polishing process accurately, and electrical control system is used to control the load process of the cylinder and rotational speed of the plates and carries.Fig.4 shows electrical control system. Fig.5 shows the driving system. The four servo motors, which connect with four transducers, are controlled by the computer through the D/A conversion. The rotation speed of the motors can be controlled by the computer control system in a programmed progress. Four rotation speed sensors are fixed in the principal axes of the four motors. The speeds of the principal axes can be fed back to the computer through these sensors and the closed loop control system is effected to assure the rotation speeds of polishing pad and the carriers are accurate.Fig.3 Schematic of the control system of electricity Fig.4 Electric control system Fig.5 Driving systemFig.6 shows pneumatic control system. The cylinder is controlled with a digital servo valve. A pressure sensor, which joints between cylinder and top plate, can feed back the pressure of the cylinder to the computer through A/D conversion. So the pressure of the cylinder can be controlled accurately. The digital servo valve is a key unit to the pneumatic control system.Fig.7 shows the structure of the digital servo valve. The digital servo valve includes some parts such as eccentric mechanism, step motor and spool etc. Eccentric mechanism is driven by step motor. The spool is driven by the eccentric mechanism. The pressure of the cylinder, which is set in process, can be realized by adjusting the position of the spool. So the pressure of cylinder can be controlled accurately by the digital servo valve. Fig.6 Pneumatic control system Fig.7 Digital servo valveThe function and characteristic of double sided polishing machine. Some characteristic in the mechanism and control system of the polishing machine as following: (a) Top plate, bottom plate, internal and external gear ring are driven by four motors respectively. Soft start and soft stop can reduce the impact influence on the workpiece and get a stabile process.(b) The process of pressure is controlled accurately with the electric digital control technology. The magnitude of pressure can be controlled with digital servo valve through the closed loop control system. (the range of load 0 kgf F 200 kgf，0.5kgf). (c) High efficiency and high quality of polishing process can be done through process parameters data-base established for different materials, which will eliminate the possible influence caused by ordinary operation effectively. Process factor of double sided polishing machining. The main factors of double sided polishing process, include the rotational speed of top plate, bottom plate, internal and external gear ring, the pressure, the characteristic of pad, slurry, environment etc., affect the polished surface quality of the wafers and the machining efficiency. How to select these parameters is the key to get the ultra-smooth surface. The pad should have appropriate rigidity and wear-resisting characteristic. Slurry, which is composed of grinding grain, water and additives, should have the function of cooling, lubrication, chip removal and non-sedimentation. The demand on the environment is of lustration, no librations and constant temperature. The pressure and speed should be stable during polishing process. 345The polishing process. Fig.8 shows the programmable polishing process. The characteristics of the process are: (a) Programmable soft start speed control prevents jolts to fragile parts, greatly reducing edge chipping.(b) Precision load ramping is gentle with fragile parts. Provides a “light touch” to thicker parts in a load, resulting in lower thickness variation.(c) Soft stop sequencing provides top plate “spin-off” to reduce or eliminate parts remaining adhered to the top plate.The choice of the polishing process affects the processing quality of the different material wafers directly. To obtain the ultra-smooth surface, the expert database provided with the optimized processes should be established.Fig.8 the programmable polishing processConclusionThe precise polishing machine has the characteristic of advanced mechanism design, computer controlled motor-movement and accurate pressure control. According to the